There exists a considerable number of chemical, biotechnological and industrial processes whose. efficiencies are dependent upon and controlled by the rates of heat exchange between flowing streams of fluids separated by impermeable barriers capable of high heat conductivity. A significant number of these processes require heat transfer between different mediums being in the same or different phases, gas-gas gas-liquid or liquid-liquid phases.
In most conventional heat exchange devices, the basic heat transfer elements, being the impermeable barriers with high heat conductivity, are built from metals or metal alloys. Conventional heat exchange devices employ basic heat transfer elements of differing geometries and flow arrangements. A survey of heat exchange devices in compact systems is found in an article by D. A. Reay entitled "Compact Heat Exchangers: A Review of Current Equipment And R & D In The Field", appearing in Heat Recovery Systems & CHP, Vol. 14, No. 5 pp. 459-474, 1994, Elsevier Science Ltd.
The advantages of heat transfer elements constructed of metals resides in their high heat conductivities and in their capabilities of working at high temperatures. The disadvantages of heat transfer elements constructed of metals lies in their high corrosivities and, due to their considerable densities, in the substantial weight of the resulting heat exchange devices.
However, there exists a broad group of applications where heat energy transfer is carried out at relatively low temperatures, such as not higher than 100.degree. C., or where direct contact of the media exchanging the heat energy with the metallic surfaces is not acceptable. Health treatment systems, biotechnology processes and food processing are important examples. These applications remain in critical need for efficient new constructions of heat energy transfer devices capable of working under low levels of driving forces, meaning, low values of temperature differences between heated and cooled media.
Recently, new developments are in great demand in environmental protection applications as well as in sea water desalination applications to make commercially availabile heat exchange devices which exhibit improved performance, higher production, lower cost of fabrication and operation, and improved corrosion resistance. Also, new designs of heat exchange devices are becoming important as components in systems utilizing membrane separation technologies, such as air separation, pervaporation, etc.
Several recent developments demonstrate that in applications where the thermal energy must be transferred between gas and liquid phases, heat exchange devices that are built from certain plastic materials have exceeded heat transfer efficiencies of heat exchange devices manufactured from metal materials. These developments have been reviewed in a recent paper entitled "Kunstoffe im Warmeaustauscherbau" by Gros, published in Kunststoffe, 84 (1994) 6, 767-771. The results presented by Gros have relatively simple physical explanation if one takes into account the fact that the performance of heat exchange devices is the result of a chain of elementary basic heat transport mechanisms taking place in flowing fluids and in separating them by an impermeable heat conductive barrier. In this chain of elementary mechanisms, the heat conductivity of the barrier that separates the energy exchanging media is only one of several influencing the total efficiency of the overall process. In addition to the heat conductivity of the barrier, the total energy transfer depends also on the rates of energy transport inside the bulk flowing fluids as well as the rate of heat flow through the fluid-solid boundaries. These elementary mechanisms combine or add together in such way that the resistances to energy flow of each individual step are in a series arrangement. Under some conditions, the overall efficiency of energy transfer in a heat exchange device can be controlled by the performance of the step with higher resistance to the heat transfer.
There exist a number of heat exchange processes where the high heat conductivity potential of metals used as construction materials in fabricating the heat exchangers are utilized to only a minor degree. It has been shown, for example, by Gros in the paper cited above, that in some constructions of gas-liquid heat exchange devices built from plastic materials, the overall heat transfer efficiencies are comparable with those of heat exchange devices constructed from metals. Heat exchange devices built from plastic materials are especially well suited in applications at ambient temperatures. As stated by Gros, plastic heat exchange devices can be of substantial advantage when applied, for example, in temperature control of flue gases in large scale flue gas cleaning systems. Other advantages follow from the better processability of thermoplastics in manufacturing in comparison with metals. A company named Serendip B.V., located in Arnnen, The Netherlands, has recently introduced plastic heat exchangers having a bundle of small diameter thin wall tubes housed in parallel relation in an outer tube. The Serendip (trademark) plastic heat exchangers are primarily for use to cool or heat corrosive liquids and to condense corrosive vapours.
While potential benefits to be gained from the use of plastic materials in the construction of heat exchange devices have been set forth by Gros, a need still exists for innovations in the design and construction of thermoplastic heat exchange devices which will provide benefits that greatly exceed those provided by metal heat exchange devices as well as recent plastic heat exhange devices and thereby overcome the considerable conventional resistance to their adoption.